The present invention relates to an electronic watch, and in particular, to an electronic analog watch driven by an ultrasonic motor.
It is known in the art to utilize ultrasonic motors to drive an electronic analog watch. Conventional electronic watches of this type use an ultrasonic motor to convert vibrational energy to rotational energy necessary to move the indicating hands by applying a traveling wave to vibrators to drive a rotor. With this type of ultrasonic motor, however, the amount of its movement inevitably involves an error. For example, where the motor is caused to rotate at the rate of 6.degree. per second, the rotation involves an error of several percent and the error is accumulated, thereby deteriorating the accuracy of the watch over time. To correct this error a rotary encoder is mounted within the watch to cancel the errors. However, in light of the requirement for miniaturization in electronic watches, the mounting of such an encoder has been difficult; increasing watch size and complexity rather than decreasing it.
Accordingly, it is desired to provide an electronic analog watch which overcomes the shortcomings of the prior art.